The present invention relates to a process for producing an aromatic carboxylic acid, which is suitable especially for producing terephthalic acid, by subjecting an alkylaromatic compound to a liquid phase oxidation with molecular oxygen.
Heretofore, an aromatic carboxylic acid, such as terephthalic acid, has been industrially produced in a large scale by a process comprising subjecting an alkylbenzene such as para-xylene, to a liquid phase oxidation in a reaction solvent containing a lower aliphatic carboxylic acid, such as acetic acid, with molecular oxygen in the presence of a catalyst constituted of a combination composed of a compound of a heavy metal, such as cobalt, manganese or the like, and a bromine compound.
When an aromatic carboxylic acid is produced in this production process, the quality of the resulting aromatic carboxylic acid is debased due to inclusion of the intermediates and contamination by impurities formed by side reactions and, in particular, large debasements occur in the hue of the powdery product thereof and in the light transmittance when it is dissolved in an aqueous solution of a base.
The object of the present invention is to provide a process for producing an aromatic carboxylic acid, in which an aromatic carboxylic acid having a low content of 4-carboxybenzaldehyde (4-CBA) and an improved hue of the powdery product thereof and exhibiting an improved light transmittance when dissolved in an aqueous solution of a base can be produced efficiently in a simple manner.
The present invention resides in the following process for producing an aromatic carboxylic acid:
(1) A process for producing an aromatic carboxylic acid by a liquid phase oxidation of an alkylaromatic compound with molecular oxygen in a reaction solvent comprising a lower aliphatic carboxylic acid in the presence of an oxidation catalyst, comprising performing the liquid phase oxidation of the alkylaromatic compound in the presence of a hydrogen gas-treated liquid containing the catalyst.
(2) The process as defined in the above (1), wherein the oxidation catalyst is constituted of a combination of a heavy metal compound and a bromine compound.
(3) The process as defined in the above (1) or (2), wherein the hydrogen gas-treated liquid is one, in which the reaction solvent containing the catalyst held in a state dissolved or dispersed therein is treated with hydrogen gas.
(4) The process as defined in any one of the above (1) to (3), wherein the amount of the hydrogen gas to be used in the treatment with hydrogen gas is 0.000001-0.05 N liter per one gram of the total sum of the reaction solvent plus the catalyst to be introduced into the oxidation reaction step.
(5) The process as defined in any one of the above (1) to (4), wherein the aromatic carboxylic acid is terephthalic acid.